Note that the interface you're famiiar with is not meant for end-users. That's not when you're supposed to use Sia.

Sia is meant to be an API used on the back of Dropbox. The big companies use Sia and their API to deliver their products. Ultimately, you will be using a cloud storage provider using Sia in clean and enticing way. YOU are not meant to be using the Sia program available. It exists to prove that we have a working product.

Oh, I am see now where source of the problem. Not users files but sia DB with renter contracts...

Actually contracts are stored in the blockchain. But only outdated versions - latest actual revisions of contracts are stored only locally on renter and host disks.
Also contracts on blockchain do not include information about on which host each piece of user data was placed on.

It is possible to restore/reconstruct all this data by querying all hosts in all active contracts renter have (and all base contracts and actual hosts addresses can be restored by scanning blockchain - only wallet/seed needed). But this functionality has not implemented yet - there are only plans for future development (ETA - next year in best case).

So backup whole sia data folder (excluding blockchain) is a very good idea for now.

At least there is no longer need to backup files uploaded to SIA itself. Lol without it (prior ver. 1.3.0) SIA was almost useless for actual data storage.

Assuming you're using the GUI application It's stored in .config in your home directory.

Take a look at ~/.config/Sia-UI
The wallet is in ~/.config/Sia-UI/sia/wallet

I think if you just delete the file/folder it should create a new one next time you start up.
Bear in mind the blockchain file is in ~/.config/Sia-UI/sia/consensus so if you remove that it'll need to be downloaded again.

This is very simple view of database, just to get some basic understanding about it contents.
Nothing processed with any logic. Only currency view as SC, Filesize and Contract statuses.
So close to raw data from sia daemon.
And a lot trash actually like unresolved = failed attempts to make contract.